The present invention relates to an apparatus for insertion and removal of releasable burnable absorber rods from the adapter plate of the top nozzle of a nuclear reactor fuel assembly.
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison rods at stationary positions in a nuclear reactor core are described in U.S. Pat. Nos. to Rose (3,361,857) and to Wood (3,510,398).
Also, the availability of assemblies of burnable absorber rods on a rapid response basis is required at reactor fuel reload time. The present design of the burnable absorber assemblies, being similar to those illustated and described in the first two patent applications cross-referenced above, includes a plurality of precisely spaced apart absorber rods and thimble plugs fastened at their upper ends to a support plate which also mounts a central hold-down device. In view of the multiplicity of components which make up the absorber assemblies and the precise spacing required between them when they are assembled together, it has been found necessary to assemble the absorber assemblies at a manufacturing facility located remote from the reactor site. The final absorber assemblies are then shipped with the fuel assemblies to the reactor site. This means that the particular absorber assembly design must be specified well in advance of the time of actual reload.
A burnable absorber assembly in which the burnable absorber rods have a releasable latching structure is illustrated and described in the fifth patent application cross-referenced above. The advantage of the releasable latching structure is that the configuration of the burnable absorber rods can be specified at the latest possible time because the assembly does not have to include the burnable absorber rods until it is installed. Thus, the nuclear reload design can be fine tuned based on the latest reactor operations input. The ultimate absorber assembly specified may advantageously include, for example, twelve burnable absorber rods and twelve thimble plugs per assembly or other combinations of absorber rods and thimble plugs.
Consequently, a need exists for a device that can be used to insert and remove the burnable absorber rods and thimble plugs from the burnable absorber assembly.